Several imaging technologies are commonly used to interrogate biological systems. Widefield imaging floods the specimen with light, and collects light from the entire specimen simultaneously, although high resolution information is only obtained from that portion of the sample close to the focal plane of the imaging objective lens. Confocal microscopy uses the objective lens to focus light within the specimen, and a pinhole in a corresponding image plane to pass to the detector only that light collected in the vicinity of the focus. The resulting images exhibit less out-of-focus background information on thick samples than is the case in widefield microscopy, but at the cost of slower speed, due to the requirement to scan the focus across the entire plane of interest.
For biological imaging, a powerful imaging modality is fluorescence, since specific sub-cellular features of interest can be singled out for study by attaching fluorescent labels to one or more of their constituent proteins. Both widefield and confocal microscopy can take advantage of fluorescence contrast. One limitation of fluorescence imaging, however, is that fluorescent molecules can be optically excited for only a limited period of time before they are permanently extinguished (i.e., “photobleach”). Not only does such bleaching limit the amount of information that can be extracted from the specimen, it can also contribute to photo-induced changes in specimen behavior, phototoxicity, or even cell death.
Unfortunately, both widefield and confocal microscopy excite fluorescence in every plane of the specimen, whereas the information rich, high resolution content comes only from the vicinity of the focal plane. Thus, both widefield and confocal microscopy are very wasteful of the overall fluorescence budget and potentially quite damaging to live specimens. A third approach, two photon fluorescence excitation (TPFE) microscopy, uses a nonlinear excitation process, proportional to the square of the incident light intensity, to restrict excitation to regions near the focus of the imaging objective. However, like confocal microscopy, TPFE requires scanning this focus to generate a complete image. Furthermore, the high intensities required for TPFE can give rise to other, nonlinear mechanisms of photodamage in addition to those present in the linear methods of widefield and confocal microscopy.
Thus, there is a need for the ability to: a) confine excitation predominantly to the focal plane of imaging optics, to reduce photodamage and photobleaching, as well as to reduce out-of-focus background; b) use widefield detection, to obtain images rapidly; and c) use linear fluorescence excitation, to avoid nonlinear photodamage.
FIG. 1 is a schematic diagram of a light sheet microscopy (LSM) system 100. As shown in FIG. 1, LSM uses a beam-forming lens 102, external to imaging optics, which includes an objective 104, to illuminate the portion of a specimen in the vicinity of the focal plane 106 of the objective. In one implementation, the lens 102 that provides illumination or excitation light to the sample is a cylindrical lens that focuses light in only one direction, thereby providing a beam of light 108 that creates a sheet of light coincident with the objective focal plane 106. A detector 110 then records the signal generated across the entire illuminated plane of the specimen. Because the entire plane is illuminated at once, images can be obtained very rapidly.
In another implementation, termed Digital Laser Scanned Light Sheet Microscopy (DSLM), the lens 102 can be a circularly symmetric multi-element excitation lens (e.g., having a low numerical aperture (NA) objective) that corrects for optical aberrations (e.g., chromatic and spherical aberrations) that are prevalent in cylindrical lenses. The illumination beam 108 of light then is focused in two directions to form a pencil beam of light coincident with the focal plane 106 of the imaging objective 104. The width of the pencil beam is proportional to the 1/NA, whereas its length is proportional to 1/(NA)2. Thus, by using the illumination lens 102 at sufficiently low NA (i.e., NA<<1), the pencil beam 108 of the excitation light can be made sufficiently long to encompass the entire length of the desired field of view (FOV). To cover the other direction defining the lateral width of the FOV, the pencil beam can be scanned across the focal plane (e.g., with a galvanometer, as in confocal microscopy) while the imaging detector 110 integrates the signal that is collected by the detection optics 112 as the beam sweeps out the entire FOV.
A principal limitation of these implementations is that, due to the diffraction of light, there is a tradeoff between the XY extent of the illumination across the focal plane of the imaging objective, and the thickness of the illumination in the Z direction perpendicular to this plane. In the coordinate system used in FIG. 1, the X direction is into the page, the Y direction is in the direction of the illumination beam, and the Z direction is in the direction in which imaged light is received from the specimen.
FIG. 2 is a schematic diagram of a profile 200 of a focused beam of light. As shown in FIG. 2, illumination light 202 of wavelength, λ, that is focused to a minimum beam waist, 2wo, within the specimen will diverge on either side of the focus, increasing in width by a factor of √{square root over (2)} in a distance of zR=πwo2/λ, the so-called Rayleigh range. Table 1 shows specific values of the relationship between the usable FOV, as defined by 2zR, and the minimum thickness 2wo of the illumination sheet, whether created by a cylindrical lens, or by scanning a pencil beam created by a low NA objective.
TABLE 12wo (μm, for λ = 500 nm)2zR (μm, for λ = 500 nm)0.20.060.40.250.60.570.81.001.01.572.06.285.039.310.015720.0628
From Table 1 it can be seen that, to cover FOVs larger than a few microns (as would be required image even small single cells in their entirety) the sheet thickness must be greater than the depth of focus of the imaging objective (typically, <1 micron). As a result, out-of-plane photobleaching and photodamage still remain (although less than in widefield or confocal microscopy, provided that the sheet thickness is less than the specimen thickness). Furthermore, the background from illumination outside the focal plane reduces contrast and introduces noise which can hinder the detection of small, weakly emitting objects. Finally, with only a single image, the Z positions of objects within the image cannot be determined to an accuracy better than the sheet thickness.